Radar sensors are used in motor vehicles in connection with driver assistance systems for the purpose of locating objects in the environment of the vehicle, in particular vehicles driving ahead, and for the purpose of measuring their distances and relative speeds. On the basis of the location data of such a radar sensor, an ACC system (adaptive cruise control), for example, not only is able to automatically regulate the speed of the own vehicle to a specific desired speed, but if a preceding vehicle has been located, also to automatically adapt the own vehicle to the speed of the vehicle driving ahead, so that this vehicle is followed at a suitable safety distance. Another application example for radar sensors in motor vehicles is a predictive safety system (PSS), which detects a looming collision as early as possible and automatically initiates measures by which the collision is averted, if possible, or by which the consequences of the collision are mitigated to the greatest extent possible.
Radar sensors in which one and the same antenna element is used both for transmitting the radar signal and also for receiving the signal reflected at an object are referred to as monostatic radar sensors. An associated mixer mixes a portion of the transmitted signal with the received signal and thereby generates an intermediate frequency signal whose frequency corresponds to the frequency difference between the transmitted and the received signal. Because of the Doppler effect, this frequency difference is a function of the relative speed of the reflecting object. In the case of a radar system in which the frequency of the transmitted signal is modulated continuously, e.g., in an FMCW radar (frequency modulated continuous wave), the frequency difference is also dependent on the propagation time of the signal, so that the use of known evaluation technologies makes it possible to derive from the location signal of an object both its distance and the relative speed.
Moreover, a multi-beam radar sensor having a plurality of transmission and receiving antennas that differ in their azimuthal directivity characteristic allows a determination of the azimuth angle of the located objects. To influence the directivity characteristic, for instance, an optical lens which refracts radar waves may be provided and/or it is possible to utilize diffraction and interference effects of the involved antenna elements. In a radar sensor having an optical lens, for example, the plurality of transmission and receiving antennas is situated along a horizontal line at a slight offset relative to the optical axis of the lens, so that their main radiation directions and, consequently, their main sensitivity directions, differ slightly from each other. By comparing the amplitudes and/or phases of the signals received from the different paths (i.e., from the different transmission and receiving antennas), it is then possible to determine the azimuth angle of the object, i.e., the directional angle under which the object is seen from the direction of the sensor.
In the case of advanced radar systems for motor vehicles, the system must respond not only to moving objects, i.e., in particular to other moving vehicles, but also to stationary objects such as stopped vehicles or other obstacles on the road. While moving vehicles are relatively easy to detect based on their own motion and the corresponding difference between the measured relative speed and the driving speed of the own vehicle, it is much more difficult to evaluate the relevance of the objects to the particular task of the driver assistance system in the case of stationary objects. In an environment rich in structures, e.g., in city driving, it must also be taken into account that a considerable number of stationary objects will generally be located within the viewing range of the radar sensor, which complicates the evaluation of the multitude of signals considerably.
For these reasons it is desirable, if possible, to set up the visual range or the field of view of the radar sensor in such a way that from the outset, if possible, only signals from objects are received that are actually also relevant to the task to be performed by the driver assistance system. In particular, the field of view should therefore be set up in such a way that, if possible, no radar echoes are received from objects located far beyond the traffic lane. On the other hand, a radar sensor (LRR: long range radar) for an automatic cruise control system (ACC), for instance, should have the largest possible range, on the order of magnitude of approximately 200 m or more. Because of the unavoidable divergences of the radar beams, at greater distances it will then be nearly impossible to restrict the field of view to the immediate road lane area.
In the case of a multi-beam radar, e.g., in a radar sensor having four transmission and receiving antennas, it is described in DE 10 2004 030 755 to symmetrically reduce the transmission power of the two outer antennas in comparison with the transmission power of the two inner antennas. In this manner a large range is able to be achieved by the two inner antennas, and a still relatively small field of view for large distances, while the weaker margin radiation ensures a sufficient width of the field of view at shorter distances.
The transmission power of the different beams is able to be set via the configuration of the associated mixers, for instance. Radar sensors of the type examined here typically use transfer mixers in which a portion of the transmitted signal used for the mixing with the received signal is transferred to the line leading to the antenna and therefore is emitted via the antenna. On the other hand, so-called isolation mixers are known where virtually no transfer of the transmitted signal to the antenna line takes place. The antenna to which such a mixer is assigned is therefore only able to receive signals transmitted from other antennas, and these received signals are then mixed with the transmitted signal in the mixer. Any possible graduation between pure transfer mixers and pure isolation mixers is possible in this context. The relative strength of the signal transferred onto the antenna line will be called the “transfer output” here.